This disclosure is related to ion sources, and more particularly to high intensity ion sources.
Ion sources may be used to generate ion beams useful in a number of applications. For example, the beams may be used to bombard targets to drive nuclear reactions for the production of isotopes.
Some ions sources ionize neutral targets via collisions with energetic electrons. Electron Cyclotron Resonant (ECR) plasma sources generate energetic electrons by exciting the cyclotron motion of the electrons within a magnetic field. The ECR plasma source is located in a vacuum chamber to control the gas that is ionized, and to reduce the pressure allowing the electrons to reach ionization energy.
A charged particle placed in a uniform magnetic field will gyrate around the magnetic field with a frequency given by the electron cyclotron frequency.
      ω    ce    =      eB    m  
If the magnetic field is not uniform, the electron still gyrates around the magnetic field, but the orbit and frequency become somewhat more complicated. ECR ion sources do not require a uniform magnetic field to operate. In fact, many of them operate in highly non-uniform magnetic fields. By applying an oscillating electric field, the cyclotron motion of the electrons can be excited. If the oscillating electric field is resonant with the electron cyclotron frequency and couples to the electron motion, the electrons will gain energy. The highest coupling would be an electric field that rotated about the magnetic field in the same direction as the electrons, and at the same rate. This electric field would look like a DC electric field in the frame of the electrons. Good coupling can also be obtained with a linear polarized electric field that oscillates perpendicular to the magnetic field. As the electrons gain energy they will collide with any gas within the source, ionizing the background gas. This forms plasma and creates more electrons that can ionize more background gas. This prior art process continues as the plasma density increases until losses are balanced with production.
In prior art devices, the balance occurs long before a powerful beam can be generated, and a powerful beam is what is needed to strike and transmute target material into such things as useful medical isotopes. Many isotopes are, in theory, thought to be useful, but heretofore their small obtainable quantities and short half lives, prevent their use.